In recent years, lithium secondary batteries are widely used as power sources for electronic devices such as mobile telephones and notebook computers, or for electric cars or electric power storage. Particularly recently, there is a rapidly increasing demand for a high capacity and high power battery with a high energy density, which can be mounted in hybrid cars or electric cars.
Lithium secondary batteries are primarily composed of a positive electrode and a negative electrode, which contain materials capable of absorption and desorption of lithium, and a non-aqueous electrolyte solution containing a lithium salt and a non-aqueous solvent.
Examples of positive electrode active materials used in a positive electrode include lithium metal oxides such as LiCoO2, LiMnO2, LiNiO2, and LiFePO4.
Furthermore, as the non-aqueous electrolyte solution, solutions prepared by mixing a mixed solvent (non-aqueous solvent) of carbonates such as ethylene carbonate, propylene carbonate, ethylene carbonate or methyl carbonate, with a Li electrolyte such as LiPF6, LiBF4, LiN(SO2CF3)2 or LiN(SO2CF2CF3)2, are used.
On the other hand, as the active material for a negative electrode that is used in negative electrodes, metal lithium, metal compounds (elemental metals, oxides, alloys with lithium, and the like) capable of absorption and desorption of lithium, and carbon materials are known. Particularly, lithium secondary batteries employing cokes, artificial graphite or natural graphite, which are all capable of absorption and desorption of lithium, have been put to practical use.
Among the battery performances, particularly in relation to lithium secondary batteries for automotive applications, an increase in output power and an increase in service life are required. It has been a considerable challenge to achieve a balance between a reduction of the resistance of a battery under various conditions and an enhancement of the service life performance of a battery.
One of the factors known to cause an increase in the resistance of a battery is a passivation film based on a solvent decomposition product or an inorganic salt, which is formed on the surface of a negative electrode. In general, it is known that since lithium metal is present among the negative electrode active material under the charging conditions, a reductive decomposition reaction of the electrolyte solution occurs at the surface of the negative electrode. In a case in which such reductive decomposition continuously occurs, the resistance of the battery increases, the charge-discharge efficiency decreases, and the energy density of the battery decreases. Furthermore, on the other hand, it is also known in regard to the positive electrode that a deterioration reaction occurs over time, the resistance continually increases, and a decrease in the battery performance is caused. In order to overcome these problems, attempts have been made to add various compounds to electrolyte solution.
As a trial for the purpose, it has been attempted to improve the battery performance by incorporating various cyclic sulfate compounds (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-189042, JP-A No. 2003-151623, JP-A No. 2003-308875, JP-A No. 2004-22523, and JP-A No. 2005-011762).